The G&P hopup unit at Airsoftparts is the hopup unit body, gears and pins. It does not (don't think so) come with a hopup rubber (and the little hopup rubber nub)...those are sold separately.
If you need a hopup unit...the G&P are fine, the KA Enhanced ones are good too (at least they've really worked well for me). Some guys from out west might chime in and go on about how they've had horrible experiences with one and will never use them again.
If you need a hopup rubber...the softer ones are for less than 380-400 and the harder ones are for 400+. I'd put a guarder black in a 400+ setup.